This invention relates to working fluids used as refrigerants for room air conditioners, car air conditioners (air conditioners for automobiles), refrigerators, freezers and the like.
Conventionally, halogenated hydrocarbons called freons have been known as working fluids for appliances such as room air conditioners, car air conditioners, refrigerators and freezers, and they are used at a condensing temperature and/or an evaporating temperature in the range of -15 to 50.degree. C. Among others, HCFC-22 (chlorodifluoromethane) is the commonest refrigerant for use in domestic room air conditioners and building air conditioners, the specified freon CFC-11 (trichloromonofluoromethane) is the commonest refrigerant for use as a foaming agent and a propellant and for use in turborefrigerating machines and large-sized refrigerators, and CFC-12 (dichlorodifluoromethane) is the commonest refrigerant for use in reciprocating compression refrigerating machines. Up to the present, these specified freons have been widely used as working fluids for refrigerating and air conditioning applications such as air conditioners, electric refrigerators, car coolers, window coolers, train air conditioners, food refrigerating plants and factory cooling systems.
However, since these specified freons severely destroys the ozone layer in the stratosphere, their production was totally abolished in the January of 1996 under an international treaty. Consequently, the substitute freon HCFC-22 is being provisionally used. However, since HCFC-22 has an ODP [i.e., ozone depletion potential based on CFC-11 (=1.0)] of 0.055, its production will be gradually decreased from 2004 and totally abolished in 2030. In place of this, substitute freons obtained by replacing the chlorine of halogenated hydrocarbons with hydrogen, such as HFC-134a (1,1,1,2-tetrafluoroethane), has already been practically used as refrigerants for car air conditioners, domestic electric refrigerators and the like. HFC-134a, which has an ODP of 0, is chiefly used as a substitute freon all over the world and is now being produced by major substitute freon makers in the world. However, HFC-134a is recently attracting attention in that its HGWP [i.e., global warming potential based on CFC-11 (=1.0)] is as high as 0.28, and this problem was discussed at the Kyoto Conference (COP3). In this connection, the HGWP of HFC-134a is said to be more than several thousand times as high as that of carbon dioxide.
In view of these circumstances, the present inventors carried on intensive investigations in order to develop a new working fluid which not only functions satisfactorily as a working fluid, but also exerts little influence on ozone layer depletion and global warming. As a result, it has been found that HFC-152a (1,1-difluoroethane) has an ODP of 0 and an HGWP of 0.03 and hence serves as a working fluid meeting the above-described requirements. Moreover, this HFC-152a has an atmospheric life of 1.5-1.7 years, which is much shorter, for example, than the atmospheric life of HFC-134a (i.e, 15 years). Also from this point of view, HFC-152a can be a working fluid which is gentle to the global environment. What is better is that HFC-152a has a high refrigerating capacity and a high coefficient of performance, and may be used as a working fluid in an amount equal to 60-70% or less of that of HFC-134a. Thus, HFC-152a exerts little influence on global warming and is also excellent from an economic point of view.
However, this HFC-152a has the only problem with safety, in that its explosive range is from 4.6 to 16.9% by volume. Recently, the use of a combustible gas (e.g., butane) as a working fluid has been being investigated in Europe and the like, and has been put into practice to some extent. When a combustible gas is used as a working fluid, conventional appliances such as air conditioners, refrigerators and freezers cannot be directly used without considerable alterations such as an increase in equipment size. Paying attention to this point, the present inventors have searched for a technique which makes it possible to use conventional appliances such as air conditioners, refrigerators and freezers without any alteration and to secure excellent safety.
In order to improve safety, German Patent No. DE4116274 has proposed the idea that 4 to 25% by weight of an inert gas (e.g., carbon dioxide) may be added to HFC-152a. However, no detailed data on its practice are disclosed and this technique remains obscure. In fact, it is difficult to improve the safety of HFC-152a solely by adding thereto a low concentration of an inert gas. On the other hand, if a high concentration of an inert gas is added thereto, its performance as a working fluid will be markedly reduced. Thus, the mere idea of adding an inert gas does not make it possible to use HFC-152a safely and efficiently as a working fluid.
According to the same concept, Japanese Patent Provisional Publication No. 67870/'96 has proposed that the combustibility problem of HFC-32 (difluoromethane) can be solved by adding a slight amount of carbon dioxide to a combination of HFC-32 and HFC-134a. However, this can only solve the problem concerning the combustibility of leakage gas at low temperatures, and fails to secure overall safety. For example, no mention is made of the safety in the event of leakage of all the working fluid composition, and this working fluid cannot necessarily regarded as safe.
Moreover, these reports rely solely on an idea or theory, and it is highly questionable whether the proposed working fluids may be directly applied to conventional appliances such as air conditioners, refrigerators and freezers in current use. Thus, these working fluids are considered to have demerits, for example, in that they require considerable alterations in the appliances. That is, although safety can be enhanced by the addition of an inert gas such as carbon dioxide, the resulting working fluid is impractical if its refrigerating capacity and coefficient of performance are low. Accordingly, it is an important problem to define the composition of a working fluid. The working fluid so defined will be economically useful if it can be directly applied to conventional appliances such as air conditioners, refrigerators and freezers in current use.